Fix style of literal creators (#637)

* Fix style of literal creators
Literal creators for UInt, SInt and Bool were declared with parens, but
virtually all uses of these methods do not use parens.  This is for
issue #539.
This fix is an API breaking change. If anyone has used parens, e.g.
val x = 1.U()
This will now be an error

* remove trailing parens from literal creators in IntegerMathTester